Structure of a ball bump for wire bonding and the formation thereof

ABSTRACT

The ball bump mainly includes a body and a protrusion. The protrusion is located at the upper of the body and essentially consists of a flat upper surface with an annular inclination. The flat upper surface and the annular inclination together define the area for wire bonding. The method of the formation of the ball bump of wire bonding mainly comprises steps of: an end of a wire held by a bonding machine is melted to form a ball; the bonding machine bonds the ball onto the bonding pad to form a ball bump; the bonding tool moves upward a predetermined vertical distance and the clamp of the bonding tool is then opened; the bonding tool is moved a predetermined horizontal distance to reduce the connection part of wire connecting to the upper of the ball bump for the convenience of wire cutting, and this forms a protrusion on the ball bump consisting of a flat upper surface with an annular inclination; the bonding tool is again moved upward a predetermined vertical distance, and the clamp releases the wire for the ball bump process which follows; the clamp is closed to hold the wire and is moved upward to pull the wire, and the wire is cut along the heat affected zone and leaves a smaller tip on the ball bump.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention is related to the structure of a ball bumpof wire bonding and the formation thereof and more particularly to theformation of the ball bump with a bonding portion that provides a flatupper surface for wire bonding.

[0003] 2. Description of the Related Art

[0004] To avoid the destruction of the bonding pads during wire bondingonto the bonding pad of the semiconductor device, the ball bump isarranged on the bonding pad in such a way as to protect it. Although theball bump protects the bonding pad directly from being struck, wirebonding on an irregular upper surface of the ball bump increases thevariability of results. Providing that a bonding machine uses the sameset of wire bonding parameters, an irregular upper surface reduces thequality and reliability of wire bonding of the semiconductor device.

[0005] The prior art of the U.S. Pat. No. 5,858,149, issued on Jan. 12,1999 to Seo et al., discloses the structure of a ball bump and theformation thereof. As shown in FIG. 1, the chip 100 has a bonding pad101. A bonding tool 110, located above the chip 100, holds a wire 120which is melted to form a ball 121. As shown in FIG. 2, the bonding tool110 strike the ball onto the bonding pad 101 of the chip 100 such thatthe bottom of the ball adheres to the bonding pad 101. Then, the clamp111 of the bonding tool 110 is opened and the bonding tool 110 pressesthe ball bump 130. As shown in FIG. 3, the bonding tool 110 is movedupward a predetermined distance so as to reserve an appropriate distanceof wire 120. The clamp 111 is then closed to hold the wire 120, and thebonding tool 110 is moved upward to pull the wire 120. The ball bump 130and the wire 120 are cut along the heat-affected zone and leave the ballbump 130 on the bonding pad 101. However, the upper of the ball bump 130forms a tip 131 whose height is determined by the heat affected-zone ofthe wire 120; so tip 131 varies in accordance with the heat-affectedzone and increases the variability of wire bonding. When the bondingtool 110 directly pushes the tip 131 in a horizontal direction, thisresults in the collapse and shift of tip 131, and the irregular uppersurface of the tip 131 increases the variability of the ball bump 130.

[0006] The present invention intends to provide a structure for the ballbump which includes a bonding portion on which a flat upper surface ofuniform height is formed so as to reduce the variability of wire bondingin such a way as to mitigate and overcome the above problem.

SUMMARY OF THE INVENTION

[0007] The primary objective of this invention is to provide a structureof the ball bump for wire bonding and the formation thereof, to form aflat upper surface on the bonding portion of the ball bump for wirebonding. By eliminating the irregularities in the structure of the ballbump, the quality and reliability of wire bonding is increased.

[0008] The secondary objective of this invention is to provide astructure for the ball bump of wire bonding and the formation thereof,such that the tip of the ball bump on the flat upper surface of thebonding portion is smaller and more uniform in structure thus theregular structure of the ball bump increase the quality and thereliability of the wire bonding.

[0009] The present invention defines the structure of the ball bump ofwire bonding. The ball bump mainly includes a body and a protrusion. Theprotrusion is located at the upper of the body and essentially consistsof a flat upper surface with an annular inclination. The flat uppersurface and the annular inclination together define the wire bondingarea.

[0010] The present invention is a method for the formation of the ballbump for wire bonding. The formation of the ball bump for wire bondingmainly comprises steps of: the end of a wire held by a bonding machineis melted to form a ball; the bonding machine bonds the ball onto thebonding pad to form a ball bump; the bonding tool is moved upward apredetermined vertical distance and the clamp of the bonding tool isthen opened; the bonding tool moves a predetermined horizontal distanceto decrease the connection part of wire connecting to the top of theball bump for the convenience of wire cutting, this creates a protrusionfrom the ball bump consisting of a flat upper surface with an annularinclination; the bonding tool is again moved upward a predeterminedvertical distance, and the clamp is opened to release the wire for theball bump process which follows; the clamp is then closed to hold thewire and moved upward to pull the wire; the wire is cut in theheat-affected zone therefore leaving a smaller tip on the ball bump.

[0011] Other objectives, advantages and novel features of the inventionwill become more apparent from the following detailed description andthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The present invention will now be described in detail withreference to the accompanying drawings herein;

[0013]FIG. 1 is a side view of the first step of ball bump formation inaccordance with the prior art;

[0014]FIG. 2 is a side view of the second step of ball bump formation inaccordance with the prior art;

[0015]FIG. 3 is a side view of the third step of ball bump formation inaccordance with the prior art;

[0016]FIG. 4 is a side view of the first step of ball bump formation inaccordance with the embodiment of the present invention;

[0017]FIG. 5 is a side view of the second step of ball bump formation inaccordance with the embodiment of the present invention;

[0018]FIG. 6 is an enlarged view of the bonding tool of FIG. 5 inaccordance with the embodiment of the present invention;

[0019]FIG. 7 is an enlarged view of the bonding tool of FIG. 5 inaccordance with the embodiment of the present invention;

[0020]FIG. 8 is a side view of the third step of ball bump formation inaccordance with the embodiment of the present invention;

[0021]FIG. 9 is a side view of the fourth step of ball bump formation inaccordance with the embodiment of the present invention;

[0022]FIG. 10 is a side view of the fifth step of ball bump formation inaccordance with the embodiment of the present invention; and

[0023]FIG. 11 is a side view of the sixth step of ball bump formation inaccordance with the embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The formation of the ball bump of wire bonding mainly comprisessteps of: the end of a wire held by a bonding machine is melted to forma ball; the bonding machine bonds the ball onto the bonding pad to forma ball bump; the bonding tool is moved a predetermined vertical distanceand the clamp of the bonding tool is opened; the bonding tool is thenmoved a predetermined horizontal distance to reduce the connection partof the wire connecting to the upper of the ball bump for the convenienceof wire cutting, thus forming a protrusion of the ball bump whichconsists of a flat upper surface with an annular inclination; thebonding tool is again moved a predetermined vertical distance, the clampreleases the wire for the ball bump process which follows; the clamp isclosed to hold the wire and moves upward to pull the wire, so the wireis cut along the heat affected zone of the connection part and thereforethe wire leaves a smaller tip on the ball bump.

[0025] Referring to FIG. 4, a capillary 210 of the bonding tool holdsthe wire 220 at an appropriate length and the end of the wire 220 ismelted to form a ball 221 for ball bump bonding. Then the clamp 211 isopened.

[0026] Referring to FIGS. 4, 5, 6 and 7, the capillary 210 of thebonding tool moves downward onto a bonding pad 201 of the chip 200 toform a ball bump 230. Because the capillary 210 has an annularinclination, the ball 221 is mechanically pressed to form the ball bump230, and the wire 220 remains connected to the ball bump 230.

[0027] Referring to FIG. 6, the method of formation of the ball bump230, in accordance with the embodiment of the present invention, appliesa capillary 210 which has an opening with the height “h” and thediameter “d”. The height “h” is 0.3-1.0 mil and the diameter “d” is1.3-1.5 mil.

[0028] Referring to FIGS. 6 and 8, the capillary 210 is moved verticallya height “H” leaving an annular inclination on a upper of the ball bump230. When the height “H” is equal to the height of the ball bump 230,the clamp 211 of the capillary 210 is opened.

[0029] Referring to FIGS. 6 and 9, after the capillary 210 movedvertically a height “H” and stopped, the capillary 210 is moved adistance “D”, called smooth distance, in a horizontal direction thusformation a protrusion with a flat upper surface for wire bonding. Thewire 220 is pushed to the side by the capillary 210 to reduce theconnection part of the wire 220 for the convenience of wire cutting. Thedistance “D”, as well as the width of the protrusion, is preferably ⅔-⅞of the diameter of the wire 220 and can be adjusted according to the gapbetween the wire 220 and the opening of the capillary 210.

[0030] Referring to FIGS. 10 and 11, the capillary 210 is again movedvertically upward a predetermined distance from the ball bump 230. Theclamp releases the wire 220 for the ball bump process which follow. Whenthe wire 220 is cut off from the ball bump 230, a length of the wire 220is reserved on the capillary 210 and is melted to form a ball. Thecapillary 210 is moved upward in a vertical direction from ball bump230. The wire 220 is cut off along the heat-affected zone so as toprovide a smaller tip on the flat upper surface 233.

[0031] Referring to FIG. 11, the ball bump 230 of the present inventionmainly includes a body 231 and a protrusion 232. The protrusion 232 islocated at the upper of the body 231 and essentially consists of a flatupper surface 233 with an annular inclination 234. The flat uppersurface 233 and the annular inclination 234 form the upper surface andside surface of the protrusion 232, respectively. The smooth surface andthe uniform level of the protrusion 232 together provide a regularstructure for wire bonding, and a tip 235 is left between the flat uppersurface 233 and the annular inclination 234. Even though the smaller tip235 is left on the protrusion 232, the variability of wire bonding onthe protrusion 232 is less than usual.

[0032] Comparing FIG. 3 with FIG. 11, the ball bump 130 of the prior artwith the tip 131 reduces the bondability and reliability of the wirebonding; but the ball bump 230 of the present invention with its flatupper surface 233 and the annular inclination increases the bondabilityand reliability of wire bonding. Therefore, the wire bonding of thepresent invention increases the bondability and reliability by using aset of parameters.

[0033] The body and protrusion of the ball bump of the present inventionhave a uniform total height, under proper controls. Thus the presentinvention provides a uniform height of balls on a substrate. Therefore,the present invention can be applied to flip chip bonding in such a waythat the protrusions of the ball bump are aimed and attached to thecorresponding bonding pad of the chip and then reflow to electricallyconnect to the corresponding bonding pad.

[0034] Although the invention has been described in detail withreference to its presently preferred embodiment, it will be understoodby one of ordinary skill in the art that various modifications can bemade without departing from the spirit and the scope of the invention,as set forth in the appended claims.

What is claimed is:
 1. A method of formation of a ball bond of wirebonding comprising steps of: an end of a wire held by a bonding machineis melted to form a ball; the bonding machine which bonds the ball ontoa bonding pad to form a ball bump; the bonding tool which is movedupward a predetermined vertical distance to form an annular inclinationon a top of the ball bump, and a clamp of the bonding tool is thenopened; the bonding tool which is moved a predetermined horizontaldistance to reduce the connection part of wire for the convenience ofwire cutting, and this forms a protrusion on the ball bump consisting ofa flat upper surface and an annular inclination; the bonding tool whichis again moved upward a predetermined vertical distance, and the clampreleases the wire for the ball bump process which follows; and the clampwhich is closed to hold the wire and moved upward to pull the wire, andthe wire is cut along the heat-affected zone.
 2. The method of formationof the ball bond of wire bonding as defined in claim 1 , wherein thebonding tool is moved upward in vertical direction to a specified heightto form the annular inclination on a upper of the ball bump; the heightshould be equal to the height of an inclination of an opening in thebonding tool.
 3. The method of formation of the ball bond of wirebonding as defined in claim 1 , wherein the bonding tool is moved apredetermined horizontal distance to form a protrusion on the ball bump,whose width is preferably ⅔ - ⅞ of the diameter of the wire.
 4. Astructure of a ball bump of wire bonding comprising: a body, and aprotrusion located at the upper of the body and essentially consistingof a flat upper surface with an annular inclination which togetherdefine an area for wire bonding. whereas the flat upper surface and theannular inclination provide a regular structure of the ball bump andthus reduce the variability of wire bonding.
 5. The structure of a ballbump of wire bonding as defined in claim 4 , wherein the height of theprotrusion of the ball bump is equal to a height of an opening of thebonding tool.
 6. The structure of a ball bump of wire bonding as definedin claim 4 , wherein the width of the protrusion is equal to ⅔-⅞ of thediameter of a wire.
 7. The structure of a ball bump of wire bonding asdefined in claim 4 , wherein the protrusion further has a smaller tip atthe edge of the flat upper surface of the protrusion.